Dry-wet cycle and chloride ion (Cl^-) penetration are the main factors affecting the durability of marine concrete. To study the influences of different environmental conditions and the addition of nano-particles on the Cl^-penetration resistance of marine concrete, nano-SiO₂ and nano-Fe₃O₄ were respectively added into ordinary concrete with different amounts, and the contrast test of Cl^-penetration was carried out by dry-wet cycle and full immersion, then the Cl^- content in concrete at different depths by chemical titration was measured. The test results show that dry-wet cycle accelerates the Cl^-migration to the inner of concrete, which makes that the total Cl^- content, free Cl^- content and bound Cl^- content in concrete and the Cl^- binding capacity of concrete under dry-wet circle condition are all higher than those under full immersion condition at the same age, and the difference in Cl^- content between the two environmental conditions becomes more and more larger with the increasing number of dry-wet cycles. Under the two environmental conditions, the total Cl^- content and free Cl^- content in concrete with nano-particles are all lower than those in ordinary concrete, but the bound Cl^- content and the Cl^- binding capacity are all higher than those in ordinary concrete. The optimum amount of two nano-particles in marine concrete is 2%, and the Cl^-penetration resistance of concrete with nano-SiO₂ is superior to that of concrete with the same amount of nano-Fe₃O₄. The surface effect and filling effect of nano-particles improve the pore structure of concrete, resulting in the Cl^-migration in concrete difficult. Both nano-SiO₂ and nano-Fe₃O4 with different amounts can enhance the chemical binding capacity and physical adsorption capacity of hydrated products to Cl^- in different extent, which can reduce the free Cl^- content in concrete to improve the Cl^- penetration resistance of marine concrete. This study can provide references for the durability design of marine concrete.